As an air flowmeter installed in an electronically-controlled fuel injection system of an internal combustion engine of an automobile or the like for measuring intake air flow, a thermal fluid flow sensor has been the mainstream because it can directly detect a mass air volume. In particular, a thermal fluid flow sensor manufactured using a semiconductor micromachining technology has received attention because it can be manufactured at low cost and can be driven by low power.
For example, a thermal micro flow sensor is disclosed, which is provided with a heater line formed to have a thermal insulation structure on a semiconductor substrate by a semiconductor micromachining technology with using a material in a range of 3.1×10−7 to 2×10−2 Ωm so that a resistance value becomes approximately 1 kΩ (for example, paragraphs [0027] to [0029] of Japanese Patent Application Laid-Open Publication No. 8-54269). Also, a flow sensor is disclosed, in which an upstream-side resistance temperature detector, a heat element, and a downstream-side resistance temperature detector are sequentially arranged in a fluid flowing direction, the resistance temperature detectors are formed of a thin film or a line made of a material having a large resistance temperature coefficient such as Ni, and the heat element is formed of Pt, which has a resistance temperature coefficient smaller than that of Ni (for example, paragraphs [0012] to [0015] of Japanese Patent Application Laid-Open Publication No. 5-223613). Furthermore, a thin-film resistor is disclosed, which is made of a platinum resistance film having a crystal grain diameter of 800 Å or larger formed on a surface of an insulated substrate by sputtering (for example, paragraph [0018] of Japanese Patent Application Laid-Open Publication No. 10-213470).
Still further, a heat-sensitive flowmeter is disclosed, in which an upstream-side beam temperature detector and a downstream-side beam temperature detector are formed on upstream and downstream sides of the same beam, a first electrical bridge circuit is formed by a main temperature detector and a fluid temperature detector together with a resistance element for balance adjustment, a second electrical bridge circuit is formed by the upstream-side and downstream-side beam temperature detectors together with other resistance elements for balance adjustment, and a digital memory arithmetic circuit retains the outputs of the first and second electrical bridge circuits to perform an arithmetic process for obtaining a flow velocity with zero-point correction (for example, FIG. 1 of Japanese Patent Application Laid-Open Publication No. 6-230021). Also, a heat-sensitive microbridge flow velocity meter is disclosed, in which a heat element and a main resistance temperature detector are formed on a main beam, a sub-resistance temperature detector is formed on a downstream side, a resistance temperature detector for fluid is formed on an upstream side, a first electrical bridge circuit in which the main resistance temperature detector, the sub-resistance temperature detector, and the resistance temperature detector for fluid are connected to one another is provided, a second electrical bridge circuit in which the sub-resistance temperature detector and the resistance temperature detector for fluid are connected to each other is provided, and an arithmetic process of the outputs of the first and second electrical bridge circuits is performed (for example, FIG. 1 of Japanese Patent Application Laid-Open Publication No. 6-300605). Still further, a heat-sensitive microbridge flowmeter is disclosed, in which a resistance element for room temperature calibration is provided on a substrate, a resistance element for current measurement is provided outside the substrate, an electrical bridge in which a resistance temperature detector for a heat element, a resistance temperature detector for fluid, and a resistance element for balance adjustment are connected to one another is provided, an electrical measurement circuit which measures a voltage drop value at any time and outputs it as a digital signal is provided, and an arithmetic processing circuit which performs an arithmetic process for eliminating an error due to the change in room temperature is provided (for example, FIG. 1 of Japanese Patent Application Laid-Open Publication No. 6-317440).
Still further, a heat-sensitive flow velocity meter is disclosed, in which two heat-sensitive units having the same or similar physical properties are provided, and the units are both calibrated so as to have a similar or equivalent static characteristic which is a characteristic under a windless state and to have different dynamic characteristics which are changed depending on flow velocity (for example, paragraph [0013] and FIG. 1 and FIG. 2 of Japanese Patent Application Laid-Open Publication No. 7-190822).